The present invention relates to an internal focusing telephoto lens that is suitable for a photographing camera, a video camera or the like. More particularly, the present invention relates to the telephoto lens whose view angle is about 25 degrees and F-number is about 2 through 2.8.
In general, a fast telephoto lens tends to be long in the overall length and heavy in weight. Therefore, such a fast telephoto lens often employs an internal focusing mechanism. The internal focusing mechanism reduces barycentric movement during the focusing, allows high speed focusing due to low-load to the focusing mechanism and reduces variation of aberration though the use of a floating effect, as compared with a telephoto lens that employs a whole-elements focusing mechanism.
U.S. Pat. No. 4,852,984, Japanese patent provisional publications Nos. Hei 5-157964 and Hei 9-159911 disclose the internal focusing telephoto lenses.
The publication Hei 1-154112 discloses the telephoto lenses whose F-numbers are 1.2 through 1.4 and view angles are about 30 degrees. Each of the telephoto lenses of the publication includes a great number of lenses, while chromatic aberration, particularly chromatic aberration in spherical aberration and lateral chromatic aberration, and astigmatism are not sufficiently corrected.
The publication Hei 5-157964 discloses the telephoto lenses whose F-numbers are 1.8 and view angles are about 29 degrees. Since each of the telephoto lenses of the publication varies spherical aberration, astigmatism and distortion during the focusing, it cannot insure good optical performance over the full focusing range.
The publication Hei 9-159911 discloses the telephoto lenses whose F-numbers are 2.0 and view angles are about 19 degrees. The telephoto lenses of the first through third embodiments of the publication include a great number of lenses. The lenses of the fourth and fifth embodiments are narrow in the view angle and insufficient in the correction of astigmatism.
Further, when the telephoto lenses disclosed in the publications are scaled up for applying to a large format camera using Brownie film, chromatic aberration also scales up, which makes correction of chromatic aberration be deficient.